Originally Posted by
joe greiner
I've used high-temperature silicone sealant to make flexible molds for lead casting - typically used at fireplace vents. Ordinary silicone sealant would work for concrete casting.
I make the molds against a plaster of Paris pattern, with water-soluble hand lotion as mold release. Spread the sealant about 6mm thick with no regard for smoothing the back. When cured (overnight sufficient), encase it with a plaster shell for rigidity.
One tube of sealant should cost less than $10 last time I checked.
Cheers,
Joe